1. Field of the Invention
This invention relates to integrated circuit (IC) fabrication technology, and more particularly, to an alignment-marker structure and a method of forming the same in IC fabrication, which is used for the purpose of assisting the precise alignment of a photo-mask to a wafer during a photolithographic process.
2. Description of Related Art
Photolithography is an important process in IC fabrication, which is used to transfer a predefined pattern of circuit layout from a photomask onto the wafer. As IC devices are now being fabricated at the submicron levels of integration, the photolithographic process now requires a great number of photomasks. During the photolithographic process, each photomask should be precisely aligned to the wafer; if even a slight deviation occurs in the transferred pattern, the entire wafer may be rendered unusable must then be discarded.
A conventional method for aligning a mask to a wafer is to provide a number of slots in the top surface of the wafer to serve as alignment marks on the wafer. During the photolithographic process, the mask alignment can be achieved by detecting the phase difference between the reflected light from these alignment-marker slots and the reflected light from the flat surface of the wafer to thereby obtain an alignment signal which can be used to control the precise alignment of the photomask to the wafer. The step height (i.e., the depth of these slots in the wafer surface) should be at least 200 .ANG. (angstrom) to allow the alignment signal to be adequate enough for use in the mask alignment.
In conventional semiconductor fabrication process for forming tungsten plugs in the wafer, however, conformal tungsten is also formed within the area where the alignment-marker slots are formed. The deposited tungsten then undergoes a chemical-mechanical polishing (CMP) to polish away a surface part thereof. Next, a conformal metallization layer is formed over the entire wafer, which also covers the alignment-marker areas. Subsequently, a photolithographic and etching process is performed to form a plurality of metal lines. One drawback to the use of the CMP process, however, is that it stresses the tungsten layer and therefore roughens--the surface. Moreover, the plasma sputtering used for metal deposition also roughens the tungsten layer surface. As a consequence, the alignment-marker areas are undesirably deformed, resulting in a weak alignment signal that causes the mask alignment to be imprecise.